find /tmp/test -name '*.txt' \
     -exec bash -c './thulac < "$(readlink -f {})" > "/mnt/tokenized/$(basename {})"' \;

Use find to search for files and to execute commands on the results. With `bash -c 'command'` you are able to execute multiple $().

Use `readlink -f {}` to create the full path to the result.

Use `basename {}` to strip the path from the result.